Fusion protein-bound magnetic particles for recombinant production and magnetic separation of polypeptides of interest

ABSTRACT

A fusion DNA sequence, which is obtained by fusing a gene coding for another useful protein to a fragment of a magA gene coding for a protein bound to an organic membrane for covering magnetic particles produced in cells of a magnetic bacterium AMB-1, is expressed in the magnetic bacterium to obtain the protein in a state of being bound to the magnetic particles. According to the present invention, useful proteins such as enzymes and antibodies can be stably obtained in a state of being bound to the organic membrane of the magnetic particles only by cultivating a transformed magnetic bacterium, and separating the magnetic particles produced in cells, without any necessity to perform a treatment such as immobilization. The functional protein immobilized on the magnetic particles can be magnetically controlled. Thus the function can be efficiently exhibited at a desired topical position. Magnetic particles to which a desired protein is bound can be semipermanently produced only by maintaining and cultivating an identical bacterial strain. Since the protein is produced on the magnetic particles, an objective protein can be magnetically separated and recovered in a short period of time. Thus it is possible to perform efficient separation and purification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to protein-bound magnetic particles and aproduction method thereof, as well as a novel gene, gene fragment,fusion DNA sequence, recombinant plasmid, and transformed magneticbacterium.

2. Description of the Prior Art

Proteins having biological activities such as enzymes and antibodiesimmobilized on magnetic particles can be led by magnetic means.Therefore, they can be led to local positions at which it has beenhitherto difficult to arrive. Further, they can be collected andseparated by using a magnetic force. Thus they are expected to beutilized in various industries including the fields of medicine andfermentation.

In the prior art, for example, immobilization of biologically activesubstances to magnetic particles is disclosed in Japanese PatentPublication (KOKOKU) No. 6-12994. Namely, magnetic particles areseparated from a magnetic bacterium by an alkaline treatment, and theyare treated with γ-aminopropyltriethoxysilane or glutaraldehyde, towhich a biologically active substance is chemically immobilized. Amethod is also known, in which magnetic particles are separated by anenzyme treatment from a magnetic bacterium in a state of being coveredwith an organic thin membrane comprising lipid, and a protein isimmobilized thereto after a treatment with glutaraldehyde. A method isalso known, in which a biologically active substance is immobilized onmagnetic particles by a chemical binding method by using SPDP (JapanesePre-examination Patent Publication (KOKAI) No. 5-209884).

Further, methods for measuring antigens or antibodies have beenproposed, in which an antigen-antibody reaction is performed by usingthe magnetic particles to which a biologically active substance ischemically immobilized by the aforementioned methods (JapanesePre-examination Patent Publication (KOKAI) Nos. 4-285857, 5-209884, and5-99926).

However, in any of the foregoing, it is necessary for a protein such asan enzyme, or an antibody, etc. to be chemically bound to magneticparticles. Thus problems have arisen in that a long period of time isrequired for the immobilization treatment, that the biological activityof the protein is deteriorated due to the immobilization treatment, thatthe amount of obtained immobilized protein is greatly dispersed amonglots, that the activity is also greatly dispersed, and that theimmobilized protein inevitably becomes expensive because the protein tobe used for immobilization is generally expensive.

SUMMARY OF THE INVENTION

Accordingly it is a task of the present invention to provideprotein-bound magnetic particles, etc. in which the aforementionedproblems are solved.

According to a first aspect of the present invention, there is providedan isolated and purified magA gene which codes for a protein bound to anorganic membrane for covering magnetic particles produced in cells of amagnetic bacterium AMB-1, and comprises a DNA sequence represented bySEQ ID NO: 1 defined in Sequence Listing.

In another aspect, the present invention provides an isolated andpurified MagA protein which is a protein bound to an organic membranefor covering magnetic particles produced in cells of a magneticbacterium AMB-1, and comprises an amino acid sequence represented by SEQID NO: 2 defined in Sequence Listing described below.

Taking notice of the property of the MagA protein to bind to the organicmembrane for covering the magnetic particles produced in cells of themagnetic bacterium, the present invention provides a method forimmobilizing another useful protein to the magnetic particles in a stateof a fusion protein, a method for producing useful proteins on themagnetic particles, and so on.

According to the present invention, useful proteins such as enzymes andantibodies can be stably obtained in a state of being bound to theorganic membrane of the magnetic particles only by cultivating atransformed magnetic bacterium, and separating the magnetic particlesproduced in cells, without any necessity to perform a treatment such asimmobilization. When the useful protein is a functional protein, thefunctional protein immobilized on the magnetic particles can bemagnetically controlled. Thus the function can be efficiently performedat a desired topical position.

Further, any protein can be produced on the magnetic particles byintroducing a gene coding for a desired protein into a plasmid of thepresent invention, and transforming a magnetic bacterium.

It is unnecessary to prepare an expensive protein such as an enzyme andan antibody. Magnetic particles to which a desired protein has bound canbe semipermanently produced only by maintaining and cultivating anidentical bacterial strain. The protein content and the activity do notdisperse among production lots, and there is a great merit of low cost.The magnetic particles thus obtained always contain the protein havingan identical activity in an identical amount.

Further, since the protein is produced on the magnetic particles, anobjective protein can be magnetically separated and recovered in a shortperiod of time. Thus it is possible to perform efficient separation andpurification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view for a method for preparing plasmids pKPLand pKML prepared in Example 1.

FIG. 2 is an explanatory view for a method for preparing a plasmid pUM5Aprepared in Example 1.

FIG. 3 is a map of restriction enzyme cleavage sites of the plasmidpKML.

FIG. 4 is an explanatory view for a method for preparing a plasmid pKMGprepared in Example 2.

FIG. 5 is a map of restriction enzyme cleavage sites of the plasmidpKMG.

FIG. 6 is an explanatory view for a method for preparing a plasmid pKMAprepared in Example 3.

FIG. 7 is a map of restriction enzyme cleavage sites of the plasmidpKMA.

FIG. 8 is an explanatory view for a method for preparing a plasmid pNELMprepared in Example 4.

FIG. 9 is a map of restriction enzyme cleavage sites of the plasmidpNELM.

FIG. 10 is an explanatory view for a method for preparing pNELMCprepared in Example 5.

FIG. 11 is a map of restriction enzyme cleavage sites of the plasmidpNELMC.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

magA gene

The present inventors have found the magA gene which comprises a DNAsequence represented by a base sequence as depicted by SEQ ID NO: 1 inSequence Listing described below, and codes for a protein bound to anorganic membrane containing phospholipid as a major component forcovering magnetic particles produced in cells of a magnetic bacterium.

The magA gene described above has been found, isolated, and purifiedfrom a magnetic bacterium AMB-1 (FERM P-13282) as follows.

The magnetic bacterium AMB-1 was subjected to site-nonspecificmutagenesis in genome by introducing a transposon Tn5 as a transposablegene having a drug resistance factor (Km), and a mutant deficient inmagnetic fine particle-producing function was prepared. Subsequently, abacterial strain without any magnetic response was separated by usingthe drug resistance factor (Km) as an index, and the genome wasextracted from the mutant in accordance with a method described inCurrent Protocols in Molecular Biology. After digestion with EcoRI, agene fragment containing the magA gene was isolated by Southernhybridization by using an index of hybrid formation with the transposonTn5, and purified by cloning into pUC19. As a result of gene analysis,the magA gene of 1.3 kb was obtained.

magA protein

A promoter sequence is adjacent to a sequence of the magA gene at anupstream position. The protein has its amino acid sequence as depictedin SEQ ID NO: 2 in Sequence Listing described below (depicted togetherwith the base sequence), including a hydrophilic region in a range of1st to 6th amino acid residues as counted from the N-terminal, ahydrophobic region in a range of following 7th to 380th amino acidresidues, and the second hydrophilic region in a range of following381th to 434th amino acid residues. The long hydrophobic region at themiddle is a membrane binding site, and the hydrophilic regions at theboth ends are in a state of being exposed out of the membrane. Indetail, the hydrophobic region includes short hydrophilic portions atabout 4 places. It is assumed that these portions are partially exposedout of the membrane.

This protein was isolated and purified as follows. The magneticbacterium AMB-1 was cultivated until a stationary phase, bacterial cellswere collected by centrifugation, and then cells were disrupted by usingFrench Press. Magnetic particles were extracted and purified from thedisrupted preparation by using a samarium cobalt magnet, followed byagitation for 2 hours in a 1% Tryton-10 mM Tris buffer to separate themagnetic particles from the membrane. As a result of electrophoresis(SDS-PAGE) for proteins included in the extracted preparation treated asdescribed above, a band was confirmed at a position of 46.8 kDa as knownas a molecular weight of a protein encoded by the magA gene.

As a result of search for homology with respect to the magA protein,high homology was presented with a protein which is responsible for thepotassium ion efflux mechanism of Escherichia coli. Thus the MagAprotein is considered to participate in efflux of cations. Accordingly,the magA gene was expressed in Escherichia coli, and inverted membraneliposomes were prepared after removing outer cell membrane. The effluxphenomenon of iron ion from the liposome was observed by using ironlabeled with a radioisotope. As a result, it was found that the magAprotein conducted discharge of iron ion.

The magA protein is encoded by the aforementioned magA gene, and, moregenerally, it is encoded by a DNA sequence fusing a base sequencerepresented by SEQ ID NO: 3 depicted in Sequence Listing.

Taking notice of the fact that the hydrophobic region of the magAprotein has a function to bind the protein to the organic membrane ofthe magnetic particles, and that the hydrophobic regions at the bothends are exposed out of the organic membrane, the present inventors havefound that when a DNA sequence coding for another useful protein isfused to the hydrophilic region, an obtained fusion DNA sequenceproduces the useful protein in the magnetic bacterium in a state ofbeing bound to the organic membrane of the magnetic particles.

magA gene fragment

Thus according to a second aspect of the present invention, there isprovided a magA gene fragment comprising a DNA sequence represented by abase sequence coding for a hydrophobic region in the amino acid sequenceof a MagA protein bound to an organic membrane for covering magneticparticles produced in cells of a magnetic bacterium AMB-1, thehydrophobic region being a region of 7th to 380th amino acid residuesshown in SEQ ID NO: 2 defined in Sequence Listing.

The magA gene fragment is useful as a fixing means for producing afusion protein in a state of being bound to the organic membrane.

The magA gene fragment essentially contains the aforementionedhydrophobic region. However, all or a part of the base sequence existingin the magA gene may be present at the 3'-terminal side or the5'-terminal side of the hydrophobic region.

Fusion DNA sequence

According to a third aspect of the present invention, there is provideda fusion DNA sequence comprising (a) the magA gene fragment, and (b) oneor two DNA sequences coding for one or two useful proteins fused to oneor both ends of the magA gene fragment.

The one or two DNA sequences coding for the one or two useful proteinsmay be fused to any one of the ends or both of the ends on the3'-terminal side and the 5'-terminal side of the magA gene fragment.When fused to the both ends, the DNA sequence to be introduced into the3'-terminal and the DNA sequence to be introduced into the 5'-terminalmay code for an identical protein, or may code for different proteins.The advantage of fusion to the both ends is obtained in the followingcases.

1) When a DNA sequence coding for an identical protein is introducedinto the both ends, the amount of the protein immobilized on themagnetic particles can be increased.

2) When DNA sequences coding for a complex system comprising two serialenzymes or proteins of an enzyme and a coenzyme-reproducing enzyme areintroduced into the 5'-terminal and the 3'-terminal, the reaction of theenzyme system can proceed quickly.

3) When DNA sequences coding for proteins to form subunits areintroduced into the 5'-terminal and the 3'-terminal, the subunits can beformed quickly and completely.

The position at which the DNA sequence coding for the aforementioneduseful protein is fused to the magA gene fragment is not limitedprovided that a base sequence produced at a fusion site is suitable foramino acid synthesis. The DNA sequence coding for the useful protein maybe introduced in a configuration close to the DNA sequence coding forthe hydrophobic region. The DNA sequence coding for the hydrophilicregion may be party present between the DNA sequence coding for thehydrophobic region and the DNA sequence coding for the useful protein.An artificially elongated DNA sequence may exist at one or both ends ofthe magA gene fragment so that a convenient restriction enzyme cleavagesite is formed. Especially, since the hydrophilic region on the5'-terminal side is short, all of it may exist, or this portion may beoptionally elongated. The restriction enzyme cleavage site on the5'-terminal side useful as a site for inserting a DNA sequence codingfor an objective useful protein includes a ScaI cleavage site introducedinto the magA gene. The restriction enzyme cleavage site on the3'-terminal side includes a SphI cleavage site and a DraIII cleavagesite. The SphI cleavage site and the DraIII cleavage site are located inthe hydrophilic region deviating toward the downstream side a littledistant from the hydrophobic region of the magA gene, and thus they areone of convenient sites respectively.

Recombinant plasmid

According to a fourth aspect of the present invention, there is provideda recombinant plasmid used for expressing the aforementioned fusion DNAsequence.

Namely, the present invention provides a recombinant plasmid comprisingthe aforementioned fusion DNA sequence.

The recombinant plasmid is obtained by introducing a fusion DNA sequencecomprising the magA gene fragment and a DNA sequence coding for anobjective protein into a suitable vector plasmid by means of a knownmethod.

Those belonging to families of, for example, pRK415 and pKT230 may beused as a vector to be used for preparing the recombinant plasmid. Agene is incorporated into a selected vector. The order of incorporationof genes, and restriction enzymes to be used are determined so that theyare most efficient for the vector, and that the genes are aligned in anobjective orientation. After determination of the procedure forintroducing genes, DNA is digested with a restriction enzyme. Anobjective gene fragment to be introduced is separated by conductingelectrophoresis. After that, ligation is performed by using ligase toligate DNA. If portions to be ligated of gene fragments are not fittedto one another, end portions are treated and blunt-ended, followed byligation. All of restriction enzymes, ligase, and enzymes for formingblunt ends to be used may be those commercially available. Finally, arestriction enzyme digestion pattern is confirmed to know whether or notan objective plasmid is formed.

In this aspect, the type of the gene coding for another protein to befused to the magA gene is not specifically limited.

Transformed magnetic bacterium

According to a fifth aspect of the present invention, there is provideda magnetic bacterium transformed with the aforementioned recombinantplasmid.

The magnetic bacterium may be transformed with the plasmid in accordancewith a known method. The magnetic bacterium to be used as a hostincludes, for example, microorganisms belonging to the genusMagnetospirillum (for example, bacterial strains AMB-1 (FERM P-13282),MS-1 (IFO 15272, ATCC 31632, DSM 3856), MSR-1 (IFO 15272, DSM 6361)),and microorganisms belonging to the genus Desulfovibrio (for example,bacterial strain RS-1 (FERM P-13283)).

The magnetic particles having the objective useful protein are producedin cells by cultivating the magnetic bacterium thus obtained under asuitable condition. Specifically, the objective protein is obtained in astate of being fused to the membrane-bound protein encoded by the magAgene fragment, in a state of being bound to the organic membrane forcovering the magnetic particles.

The protein-bound magnetic particles are especially useful when theprotein is a functional protein, because the magnetic particles can bemoved to a desired place by using a magnetic force, and the functionpossessed by the protein can be performed at the place.

Functional protein-bound magnetic particles

Thus, according to a sixth aspect of the present invention, there isprovided functional protein-bound magnetic particles comprising magneticparticles, and a fusion protein containing one or two functionalproteins encoded by the aforementioned fusion DNA sequence (providedthat said useful proteins is functional proteins) bound to an organicmembrane for covering surfaces of the magnetic particles.

Method for producing protein-bound magnetic particles

According to a seventh aspect of the present invention, there isprovided a method for producing the aforementioned useful protein-boundmagnetic particles, comprising the steps of cultivating a magneticbacterium transformed with a plasmid containing the aforementionedfusion DNA sequence to express the aforementioned fusion DNA sequenceand to produce the fusion protein containing a useful protein in cellsin a state of being bound to an organic membrane for covering themagnetic particles.

The useful protein-bound magnetic particles can be easily collected byutilizing a magnetic force after disrupting or lysing cells of themagnetic bacterium grown by cultivation in accordance with anconventional method.

There is no limitation on the gene coding for the useful protein fusedto the magA gene fragment according to the present invention, as well asthe protein expressed thereby. In the case of the use for a purpose ofmedicine or industries utilizing fermentation, a protein having acertain function, for example, a biological activity, will be used.However, there is no limitation thereto. The present invention can beutilized to produce proteins which are difficult to be separated andobtained by using conventional methods. Namely, such a protein can beproduced on the magnetic particles in a form of fusion protein,separated easily magnetically, and collected.

Among the useful proteins, the functional protein includes, for example,antigens, antibodies, immuno-related proteins such as Protein A,proteins having binding ability such as lectin and avidin, coenzymes,and enzymes such as hydrolases, oxidoreductases, isomerases,transferases, elimination enzymes, and restriction enzymes.

The method for producing useful proteins described above is useful as amethod for producing such proteins when the proteins are difficult to beobtained in a pure form in accordance with conventional productionmethods, even when the proteins are useful proteins having nofunctionality. In this aspect, the useful protein is obtained in a stateof being bound to the magnetic particles, however, it can be easilyseparated and purified from other cellular components in accordance witha magnetic method using a magnet or the like in the method describedabove.

Applicability

The present invention has applicability exemplified as follows.

1) Enzyme-bound carrier

In general, all enzymes which are ideally considered to work in asite-specific manner are worth while to be expressed on the surface ofthe magnetic particles. For example, when it is intended to perform anenzyme reaction locally in a reaction system of a biochemical reaction,magnetic particles on which the enzyme is expressed are useful. When itis intended to administrate an enzyme specifically to an organ in adisease of a certain enzyme system, it is possible to treat the diseaseby magnetically navigating the enzyme expressed on surface of magneticparticles.

2) DNA carrier

When a gene of a protein having an ability to bind to DNA or RNA, forexample a protein such as a repressor, is fused to a magA gene fragment,and immobilized on magnetic particles according to the presentinvention, the magnetic particles can be used as a carrier to transportthe gene. The protein such as a repressor has a property to lose itsbinding force in the presence of a specified substance. Therefore, Thegene can be transported by utilizing this phenomenon. The repressorprotein includes, for example, LacI. LacI is a protein which controlsgene expression of lactose-decomposing enzyme, which is a repressor thatsuppresses transcription of the gene by being bound to a downstreamregion of a promoter. LacI loses its ability to bind to DNA as a resultof binding to a chemical substance such as lactose and IPTG. Namely,there is a system in which expression occurs when a substrate to bedecomposed is present. There is a specic DNA domain to which LacI binds.LacI can bind to a gene into which the DNA domain has been incorporated.

For example, LacI binds to a gene into which the DNA domain has beenincorporated, by expressing LacI on the surface of the magneticparticles. On the other hand, LacI loses its ability to bind to DNA by asubstance, known as IPTG. By using these phenomena, DNA can bemagnetically transported from a particular place, and DNA can beliberated by addition of IPTG. Thus the gene can be transported to adesired place.

Therefore, the magnetic particles of the present invention are expectedto be applied as carriers for transporting genes in genetic therapy. Forexample, at present, therapeutic methods are being diligently studied inwhich antisense RNA produced from a complementary chain of an objectivegene is expressed to suppress expression of the objective gene byforming an RNA hybrid. The present invention can be used as a means forcarrying DNA which serves to express the antisense RNA.

Further, a protein having a property to bind to a metal is expressed onthe surface of the magnetic particles, and it can be used to recover ordetect the metal. Recovery and detection may be performed by magneticrecovery.

3) Protein production system

In general, proteins which are said to be difficult in separation andpurification can be easily separated and purified by expressing them asa fusion protein with the MagA protein on the magnetic particles.Because, the protein expressed on the surface of the magnetic particlescan be easily recovered finally by using a magnetic force. Consideringthe high dispersibility of the magnetic particles obtained from themagnetic bacterium, many proteins may exist which can be used in a stateof being immobilized on the magnetic particles. For example, enzymes foralcohol fermentation can be sufficiently used even in a state of beingbound to the magnetic particles if they keep enzyme activities.

EXAMPLES Example 1 I. Preparation of Recombinant Plasmids

In order to express a firefly luciferase gene (luc gene, produced byToyo Ink) in a magnetic bacterium, and produce a luminescent proteinencoded by the gene on an organic membrane for covering magneticparticles, a plasmid pKML ligated with a magA-luc fusion gene, and aplasmid pKPL ligated with only the luc gene without containing any magAgene were prepared in accordance with a method shown in FIG. 1.

(1) A plasmid pRK415 having a tetracycline resistance gene and capableof gene introduction into a magnetic bacterium AMB-1 through conjugativetransfer (N. T. Neen, S. Tamaki, D. Kobayashi, and D. Trallinger, 1988,Gene, 70: 191-197) was used as a vector. The luc gene was incorporatedinto pRK415 to prepare a plasmid pKLC. The plasmid pKLC was digested ata BamHI existing at an upstream portion of the luc gene, and blunt-endedby using Blunting Kit (produced by Takara Shuzo Co., Ltd.).

(2) Chromosome of AMB-1 was digested with EcoRI, and randomlyincorporated into λZAPII, a λDNA, to prepare a λZAPII gene bank. Thegene bank was prepared in the form of plaques by packaging genes intophage particles, followed by infection to Escherichia coli. pUM5A wasobtained by cloning 2.6 kbp of an EcoRI gene fragment containing themagA gene separated from the λZAPII gene bank into pUC19 (FIG. 2).Plaque Southern hybridization was performed upon the separation.

Next, the magA gene was digested at a SphI of the plasmid pUM5A. A magAgene fragment was separated, blunt-ended in the same manner, and ligatedwith the plasmid pKLC to prepare the plasmid pKML. Thus the luc gene canbe translated without deviating from a reading frame of magA to producea fusion protein.

(3) Only the sequence of a promoter separated by digestion with EcoRIand NcoI from the plasmid pUM5A was ligated with the plasmid pKLC havingbeen blunt-ended in the same manner, and the plasmid pKPL was therebyprepared.

Two types were prepared in accordance with the operation describedabove, namely the plasmid pKPL with only the promoter region ligatedwith the luc gene, and pKML with the ligated magA-luc fusion gene. A mapof restriction enzyme cleavage sites of pKML is shown in FIG. 3.

II. Preparation of Transconjugants

Next, the two recombinant plasmids obtained in I were introduced into awild strain AMB-1 by means of conjugative transfer to preparetransconjugants. E. coli S17-1, which was used as a donor in theconjugative transfer, had a tra gene. Thus the conjugative transfercould be performed without using any helper plasmid. Cells of themagnetic bacterium in a mid-logarithmic or late-logarithmic phasecultivated in an MSGM medium (about 8×10⁷ cells/ml) were used for theconjugative transfer. Colonies generated by introducing the plasmid onthe previous day were scraped and suspended to give 10⁹ to 10¹⁰cells/ml, which were used as donor cells. The bacteria were mixed in1:50 (magnetic bacterium:Escherichia coli), spotted on an agar plate toperform mating. After 6 hours, spots were cut with a knife to recoverbacterial cells by using about 5 ml of MSGM medium. This suspension wasinoculated to an MSGM medium added with 2.5 μg/ml of tetracycline.Cultivation was performed at 25° C., and grown cells were used astransconjugants. In this procedure, Escherichia coli does not grow onthe MSGM medium.

Next, magnetic particles were separated from the magnetic bacteriumcultivated in the MSGM medium after the conjugative transfer. Cells ofthe magnetic bacterium were collected by centrifugation, subsequentlywashed twice with 10 mM Tris buffer, suspended at a concentration not toexceed a cell concentration of 0.1 g wet cell/ml, and sonicated at anoutput of 120 W for 30 seconds five times. A disrupted cell suspensionwas treated for 30 minutes with an Nd-Co magnet abutting against anouter wall surface of a vessel while cooling it in the vessel with ice,and thus the magnetic particles were separated from the suspension. Cellmembrane components were separated by centrifuging the suspension at5,000 G for 15 minutes, and cytoplasm components were separated byultracentrifuge at 100,000 G for 1.5 hour.

The luciferase activity of each fraction of the magnetic particles, cellmembrane, and cytoplasm was determined by using a Pica Gene luminescencekit (produced by Toyo Ink), and measuring the amount of luminescencewith a luminometer. Thus the expression and the expression amount of theintroduced luciferase gene were determined. Results are shown in Table1.

                  TABLE 1    ______________________________________                      pKPL pKML    ______________________________________    Cytoplasmic fraction                        351.8   9.4    Cell membrane fraction                         26.2  129.9    Magnetic fine particle fraction                         1.9    12.5    ______________________________________     (unit: kilocounts/mg protein)

As shown in Table 1, the magnetic bacterium in which the pKPL gene withthe luc gene ligated with only the sequence of the promoter region wasintroduced, had a high amount of luminescence of the cytoplasmicfraction, and had a low protein expression efficiency on the surface ofthe magnetic particles. On the contrary, the magnetic bacterium in whichpKML ligated with the magA-luc fusion gene was introduced, provided highamounts of luminescence in the magnetic fine particle fraction and thecell membrane fraction. Namely, the magA fusion protein which is aprotein capable of binding to the membrane was expressed also in themagnetic fine particle fraction, demonstrating the production andseparation of the luc protein on the organic membrane for covering themagnetic particles.

Example 2

A gene coding for an antigen recognition site of an anti-rabbit IgGantibody (igg gene, produced by Pharmacia) was used. In order to producean antibody protein encoded by the gene on the organic membranecontaining as a major component phospholipid for covering the magneticparticles, a plasmid pKMG with a ligated magA-igg fusion gene wasprepared in accordance with a method shown in FIG. 4.

That is, the plasmid pRK415 having a tetracycline resistance gene andcapable of gene introduction into a magnetic bacterium AMB-1 throughconjugative transfer was used as a vector. pRK415 was digested withEcoRI, and blunt-ended, into which the igg gene was incorporated toprepare a plasmid pKGC.

Next, chromosome of the magnetic bacterium AMB-1 was digested withEcoRI, and randomly incorporated into λZAPII (produced by STRATAGENE), aλDNA, to prepare a ZAPII gene bank. The gene bank was prepared in theform of plaques by packaging genes into phage particles, followed byinfection to Escherichia coli. 2.6 kbp of an EcoRI gene fragmentcontaining the magA gene separated from the λZAPII gene bank was clonedinto pUC19 to prepare pUM5A.

Next, the magA gene was digested at a SphI of the plasmid pUM5A. ThemagA gene was separated, blunt-ended, and ligated with the plasmid pKGCto prepare a plasmid pKMG. A map of restriction enzyme cleavage sites ofpKMG is shown in FIG. 5.

This plasmid was introduced into a wild strain AMB-1 by means ofconjugative transfer to prepare transconjugants. The transconjugantswere cultivated, and cells were recovered and disrupted to recover themagnetic particles. The magnetic particles were used to performimmunoassay using rabbit IgG as an antigen. In the immunoassay, asandwich method was used, in which an alkaline phosphatase-labeledanti-rabbit IgG antibody was used as a secondary antibody. Fordetection, the luminescence emitted by the reaction between alkalinephosphatase and AMPPD (Boehringer Mannheim Biochemica) was measuredusing a luminometer. As a result, the antibody could be detected.According to the experiment, the antibody bound to the magneticparticles of the transconjugants as a MagA fusion protein. Thus it waspossible to produce the magnetic particles capable of being used for anantigen detecting system without any immobilizing operation.

Example 3

A Protein A gene (separated from pEZZ18 produced by Pharmacia) was used.In order to produce a protein encoded by the gene on the organicmembrane containing as a major component phospholipid for covering themagnetic particles, a plasmid pKMA with a ligated magA-Protein A fusiongene was prepared in accordance with a method shown in FIG. 6.

Namely, the plasmid pRK415 having a tetracycline resistance gene andcapable of gene introduction into a magnetic bacterium AMB-1 throughconjugative transfer was used as a vector. pRK415 was digested withEcoRI, into which the Protein A gene was incorporated to prepare aplasmid pKAC.

Next, chromosome of the magnetic bacterium AMB-1 was digested withEcoRI, and randomly incorporated into λZAPII (produced by STRATAGENE), aλDNA, to prepare a λZAPII gene bank. The gene bank was prepared in theform of plaques by packaging genes into phage particles, followed byinfection to Escherichia coli. 2.6 kbp of an EcoRI gene fragmentcontaining the magA gene separated from the λZAPII gene bank was clonedinto pUC19 to prepare pUM5A.

Next, the magA gene was digested at a SphI of the plasmid pUM5A. ThemagA gene was separated, and ligated with the blunt-ended plasmid pKACto prepare a plasmid pKMA. A map of restriction enzyme cleavage sites ofpKMA is shown in FIG. 7.

The plasmid was introduced into a wild strain AMB-1 by means ofconjugative transfer to prepare transconjugants. The transconjugantswere cultivated, and cells were recovered and disrupted to recover themagnetic particles. Taking notice of the binding ability of Protein A toIgG, the magnetic particles were mixed with anti-cedar pollen,anti-wheat, or anti-egg IgG. The mixed particles were washed, and theamount of immobilized antibody was measured. As a result, it wasrevealed that the immobilization was possible in approximately the samedegree as that of a chemical binding method using SPDP having beenhitherto used (Japanese Pre-examination Patent Publication (KOKAI) No.5-209884). As a result of experiments for detecting antigens performedin the same manner as Example 1, all of the three antigens could bedetected. Thus it was demonstrated that various IgG's could be readilyand conveniently bound to the surface of the magnetic particles by usingthe MagA fusion protein of Protein A, and that antigens could bedetected equivalently to the conventional chemical binding.

Example 4

A firefly luciferase gene (luc gene) was introduced into a 5'-terminalside of the magA gene to prepare a plasmid pNELM containing a luc-magAfusion DNA sequence in accordance with a method shown in FIG. 8.

(1) The plasmid pRK415 having a tetracycline resistance gene and capableof gene introduction into a magnetic bacterium AMB-1 through conjugativetransfer was used as a vector. An NcoI cleavage site and an EcoNIcleavage site of pRK415 were deleted by a blunt-ending treatment toprepare a plasmid pRK415NE. pRK415NE was digested with SacI to produce aplasmid pNEPL incorporated with a magA promoter-luc fusion gene.

(2) Next, primers designed for a ScaI cleavage site were used with theplasmid pUM5A harboring the magA gene. The magA gene was amplified byPCR, digested with ScaI, and ligated with the plasmid pNEPL having beensubjected to digestion with EcoNI followed by blunt end formation, toprepare a plasmid pNELM. A map of restriction enzyme cleavage sites ofpNELM is shown in FIG. 9.

(3) The plasmid was introduced into a wild strain AMB-1 by means ofconjugative transfer to prepare transconjugants. The transconjugantswere cultivated and grown. Cells were recovered and disrupted to recoverthe magnetic particles using a magnet. The luciferase activity of themagnetic particles was determined by using a Pica Gene luminescence kit(produced by Toyo Ink), and measuring the amount of luminescence with aluminometer. Results are shown in Table 2.

                  TABLE 2    ______________________________________                     pKML  PNELM    ______________________________________    Magnetic fine particle fraction                       24.5    45.8    ______________________________________     (unit: kilocounts/μg protein)

Next, in order to confirm the exposing directions at the C-terminal andthe N-terminal of the magA protein on the magnetic particles, themagnetic particles originating from transconjugants harboring theplasmid pKML ligated with the luc gene at the 3'-terminal of the magAgene, and the magnetic particles originating from transconjugantsharboring the plasmid pNELM ligated with the luc gene at the 5'-terminalof the magA gene were used to perform an immunoassay usinganti-luciferase IgG, and alkaline phosphatase-labeled anti-rabbit IgG.For detection, the luminescence emitted by the reaction between alkalinephosphatase and AMPPD (Boehringer Mannheim Biochemica) was measured byusing a luminometer. Results are shown in Table 3.

                  TABLE 3    ______________________________________                   AMB-1   pKML    pNELM    ______________________________________    Magnetic fine particle fraction                     37        290     195    ______________________________________     (unit: kilocounts/mg protein)

According to this experiment, it was revealed that the N-terminal andthe C-terminal of the MagA protein were exposed to the outside of theorganic membrane for covering the magnetic particles. Therefore, it ispossible to select the production as a fusion protein fused to theN-terminal of the MagA protein or the production as a fusion proteinfused to the C-terminal, in accordance with the property of a functionalprotein.

Example 5

In order to simultaneously express a firefly luciferase gene and achloramphenicol acetyl transferase (CAT) gene on the magnetic particles,a plasmid pNELMC as a gene containing a luc-magA-cat fusion DNA sequencewas prepared in accordance with a method shown in FIG. 10.

Namely, the plasmid pRK415 having a tetracycline resistance gene andcapable of gene introduction into a magnetic bacterium AMB-1 throughconjugative transfer was used as a vector. The NcoI cleavage site andthe EcoNI cleavage site of pRK415 were deleted by a blunt-endingtreatment to prepare a plasmid pRK415NE. pRK415NE was digested with SacIto produce a plasmid pNEPL incorporated therein with a magA promoter-lucfusion gene.

Next, primers designed for a ScaI cleavage site were used with theplasmid pUM5A harboring the magA gene. The magA gene was amplified byPCR, digested with ScaI, and ligated with the plasmid pNEPL having beensubjected to digestion with EcoNI, followed by blunt end formation, toprepare a plasmid pNELM.

Next, an SD sequence of the cat gene was eliminated, and primersdesigned for a ScaI cleavage site were used with the gene. The cat genewas amplified by PCR. It was ligated with the plasmid pNELM having beensubjected to digestion with DraIII, followed by blunt end formation toprepare a plasmid pNELMC. A map of restriction enzyme cleavage sites ofpNELMC is shown in FIG. 11.

This plasmid was introduced into a wild strain AMB-1 by means ofconjugative transfer to prepare transconjugants. The transconjugantswere cultivated and grown. Cells were recovered and disrupted to recoverthe magnetic particles using a magnet. The luciferase activity of themagnetic particles was determined by using a PICA GENE luminescence kit(produced by Toyo Ink), and measuring the amount of luminescence with aluminometer. As a result, a luciferase activity equivalent to that ofpNELM was presented.

Next, as a result of CAT assay for the magnetic particles, the CATactivity was detected. The CAT assay was performed by detectingacetylated chloramphenicol produced by the reaction betweenchloramphenicol acetyl transferase and chloramphenicol by means of thinlayer chromatography. As a result, each of the proteins bound to theboth ends of the magA gene protein exhibited an activity equivalent tothat obtained in the case of binding of each of the proteins singly.According to this experiment, it was possible to achieve simultaneousexpression on the magnetic particles by constructing a fusion gene withgenes coding for proteins having different functions ligated at the bothends of the magA gene protein. Thus the magnetic particles having anumber of functions could be produced.

    __________________________________________________________________________    SEQUENCE LISTING    (1) GENERAL INFORMATION:    (iii) NUMBER OF SEQUENCES: 3    (2) INFORMATION FOR SEQ ID NO:1:    (i) SEQUENCE CHARACTERISTICS:    (A) LENGTH: 1302 base pairs    (B) TYPE: nucleic acid    (C) STRANDEDNESS: double    (D) TOPOLOGY: linear    (ii) MOLECULE TYPE: DNA (genomic)    (ix) FEATURE:    (A) NAME/KEY: CDS    (B) LOCATION: 1..1302    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:    ATGGAACTGCATCATCCCGAACTGACCTATGCCGCCATCGTCGCCCTG48    MetGluLeuHisHisProGluLeuThrTyrAlaAlaIleValAlaLeu    151015    GCCGCCGTGCTGTGCGGCGGGATGATGACGCGCCTGAAGCAGCCGGCC96    AlaAlaValLeuCysGlyGlyMetMetThrArgLeuLysGlnProAla    202530    GTCGTCGGCTACATCCTGGCGGGGGTGGTGCTGGGACCCAGCGGCTTC144    ValValGlyTyrIleLeuAlaGlyValValLeuGlyProSerGlyPhe    354045    GGGCTGGTGAGCAACCGCGACGCCGTGGCCACCCTGGCCGAGTTCGGC192    GlyLeuValSerAsnArgAspAlaValAlaThrLeuAlaGluPheGly    505560    GTGCTGATGCTGCTGTTCGTCATCGGCATGAAGCTGGACATCATCCGC240    ValLeuMetLeuLeuPheValIleGlyMetLysLeuAspIleIleArg    65707580    TTTCTCGAAGTGTGGAAGACGGCCATCTTCACCACGGTTCTGCAGATC288    PheLeuGluValTrpLysThrAlaIlePheThrThrValLeuGlnIle    859095    GCCGGCAGCGTGGGCACGGCCCTGCTGCTGCGTCACGGCCTGGGCTGG336    AlaGlySerValGlyThrAlaLeuLeuLeuArgHisGlyLeuGlyTrp    100105110    AGCCTGGGGCTGGCGGTGGTGCTGGGCTGTGCCGTGGCGGTGTCGTCC384    SerLeuGlyLeuAlaValValLeuGlyCysAlaValAlaValSerSer    115120125    ACCGCCGTAGTGATCAAGGTGCTGGAATCCTCGGACGAGCTGGACACG432    ThrAlaValValIleLysValLeuGluSerSerAspGluLeuAspThr    130135140    CCGGTCGGCCGCACCACCCTTGGCATCCTGATCGCCCAGGACATGGCG480    ProValGlyArgThrThrLeuGlyIleLeuIleAlaGlnAspMetAla    145150155160    GTGGTGCCCATGATGCTGGTGCTGGAATCCTTCGAGACCAAGGCGCTG528    ValValProMetMetLeuValLeuGluSerPheGluThrLysAlaLeu    165170175    CTGCCCGCCGACATGGCCCGGGTGGTGCTGTCGGTGCTGTTCCTGGTG576    LeuProAlaAspMetAlaArgValValLeuSerValLeuPheLeuVal    180185190    CTGCTGTTCTGGTGGCTGTCCAAGCGCCGCATCGACCTGCCGATCACC624    LeuLeuPheTrpTrpLeuSerLysArgArgIleAspLeuProIleThr    195200205    GCCCGGCTTTCCCGCGATTCTGACCTTGCCACCCTGTCGACCCTGGCC672    AlaArgLeuSerArgAspSerAspLeuAlaThrLeuSerThrLeuAla    210215220    TGGTGTTTCGGCACCGCCGCCATCTCCGGCGTGCTGGACTTGTCGCCG720    TrpCysPheGlyThrAlaAlaIleSerGlyValLeuAspLeuSerPro    225230235240    GCCTATGGCGCCTTCCTGGGCGGCGTGGTGCTGGGCAATTCCGCCCAG768    AlaTyrGlyAlaPheLeuGlyGlyValValLeuGlyAsnSerAlaGln    245250255    CGCGACATGCTGTTGAAGCGTGCCCAGCCCATCGGCAGCGTGCTGCTG816    ArgAspMetLeuLeuLysArgAlaGlnProIleGlySerValLeuLeu    260265270    ATGGTGTTCTTCCTGTCCATCGGGCTGCTGCTCGACTTCAAGTTCATC864    MetValPhePheLeuSerIleGlyLeuLeuLeuAspPheLysPheIle    275280285    TGGAAGAATCTGGGCACCGTTCTCACCCTGCTGGCCATGGTGACCCTG912    TrpLysAsnLeuGlyThrValLeuThrLeuLeuAlaMetValThrLeu    290295300    TTCAAGACGGCGCTGAACGTCACGGCGCTGCGCCTGGCGCGGCAGGAC960    PheLysThrAlaLeuAsnValThrAlaLeuArgLeuAlaArgGlnAsp    305310315320    TGGCCCAGCGCCTTCCTGGCCGGCGTGGCCCTGGCCCAGATCGGCGAG1008    TrpProSerAlaPheLeuAlaGlyValAlaLeuAlaGlnIleGlyGlu    325330335    TTCTCGTTCCTGCTGGCCGAGACCGGCAAGGCGGTCAAGCTGATCAGC1056    PheSerPheLeuLeuAlaGluThrGlyLysAlaValLysLeuIleSer    340345350    GCCCAGGAGACCAAGCTGGTGGTGGCGGTCACCGTGCTGTCCCTGGTG1104    AlaGlnGluThrLysLeuValValAlaValThrValLeuSerLeuVal    355360365    CTGTCGCCGTTCTGGCTGTTCACCATGCGGCGCATGCACCGGGTGGCG1152    LeuSerProPheTrpLeuPheThrMetArgArgMetHisArgValAla    370375380    GCGGTGCATGTCCATTCGTTCCGCGATCTGGTCACGCGGCTGTATGGC1200    AlaValHisValHisSerPheArgAspLeuValThrArgLeuTyrGly    385390395400    GACGAGGCCCGCGCTTTCGCCCGCACCGCGCGGCGGGCCCGTGTGCTG1248    AspGluAlaArgAlaPheAlaArgThrAlaArgArgAlaArgValLeu    405410415    GTGCGGCGTGGTTCCTGGAGGGATGACCCCAATGCCGGACCTGGCTCT1296    ValArgArgGlySerTrpArgAspAspProAsnAlaGlyProGlySer    420425430    GGAATT1302    GlyIle    (2) INFORMATION FOR SEQ ID NO:2:    (i) SEQUENCE CHARACTERISTICS:    (A) LENGTH: 434 amino acids    (B) TYPE: amino acid    (D) TOPOLOGY: linear    (ii) MOLECULE TYPE: protein    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:    MetGluLeuHisHisProGluLeuThrTyrAlaAlaIleValAlaLeu    151015    AlaAlaValLeuCysGlyGlyMetMetThrArgLeuLysGlnProAla    202530    ValValGlyTyrIleLeuAlaGlyValValLeuGlyProSerGlyPhe    354045    GlyLeuValSerAsnArgAspAlaValAlaThrLeuAlaGluPheGly    505560    ValLeuMetLeuLeuPheValIleGlyMetLysLeuAspIleIleArg    65707580    PheLeuGluValTrpLysThrAlaIlePheThrThrValLeuGlnIle    859095    AlaGlySerValGlyThrAlaLeuLeuLeuArgHisGlyLeuGlyTrp    100105110    SerLeuGlyLeuAlaValValLeuGlyCysAlaValAlaValSerSer    115120125    ThrAlaValValIleLysValLeuGluSerSerAspGluLeuAspThr    130135140    ProValGlyArgThrThrLeuGlyIleLeuIleAlaGlnAspMetAla    145150155160    ValValProMetMetLeuValLeuGluSerPheGluThrLysAlaLeu    165170175    LeuProAlaAspMetAlaArgValValLeuSerValLeuPheLeuVal    180185190    LeuLeuPheTrpTrpLeuSerLysArgArgIleAspLeuProIleThr    195200205    AlaArgLeuSerArgAspSerAspLeuAlaThrLeuSerThrLeuAla    210215220    TrpCysPheGlyThrAlaAlaIleSerGlyValLeuAspLeuSerPro    225230235240    AlaTyrGlyAlaPheLeuGlyGlyValValLeuGlyAsnSerAlaGln    245250255    ArgAspMetLeuLeuLysArgAlaGlnProIleGlySerValLeuLeu    260265270    MetValPhePheLeuSerIleGlyLeuLeuLeuAspPheLysPheIle    275280285    TrpLysAsnLeuGlyThrValLeuThrLeuLeuAlaMetValThrLeu    290295300    PheLysThrAlaLeuAsnValThrAlaLeuArgLeuAlaArgGlnAsp    305310315320    TrpProSerAlaPheLeuAlaGlyValAlaLeuAlaGlnIleGlyGlu    325330335    PheSerPheLeuLeuAlaGluThrGlyLysAlaValLysLeuIleSer    340345350    AlaGlnGluThrLysLeuValValAlaValThrValLeuSerLeuVal    355360365    LeuSerProPheTrpLeuPheThrMetArgArgMetHisArgValAla    370375380    AlaValHisValHisSerPheArgAspLeuValThrArgLeuTyrGly    385390395400    AspGluAlaArgAlaPheAlaArgThrAlaArgArgAlaArgValLeu    405410415    ValArgArgGlySerTrpArgAspAspProAsnAlaGlyProGlySer    420425430    GlyIle    (2) INFORMATION FOR SEQ ID NO:3:    (i) SEQUENCE CHARACTERISTICS:    (A) LENGTH: 1302 base pairs    (B) TYPE: nucleic acid    (C) STRANDEDNESS: double    (D) TOPOLOGY: linear    (ii) MOLECULE TYPE: DNA (genomic)    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:    ATGGARYTNCAYCAYCCNGARYTNACNTAYGCNGCNATHGTNGCNYTNGCNGCNGTNYTN60    TGYGGNGGNATGATGACNMGNYTNAARCARCCNGCNGTNGTNGGNTAYATHYTNGCNGGN120    GTNGTNYTNGGNCCNWSNGGNTTYGGNYTNGTNWSNAAYMGNGAYGCNGTNGCNACNYTN180    GCNGARTTYGGNGTNYTNATGYTNYTNTTYGTNATHGGNATGAARYTNGAYATHATHMGN240    TTYYTNGARGTNTGGAARACNGCNATHTTYACNACNGTNYTNCARATHGCNGGNWSNGTN300    GGNACNGCNYTNYTNYTNMGNCAYGGNYTNGGNTGGWSNYTNGGNYTNGCNGTNGTNYTN360    GGNTGYGCNGTNGCNGTNWSNWSNACNGCNGTNGTNATHAARGTNYTNGARWSNWSNGAY420    GARYTNGAYACNCCNGTNGGNMGNACNACNYTNGGNATHYTNATHGCNCARGAYATGGCN480    GTNGTNCCNATGATGYTNGTNYTNGARWSNTTYGARACNAARGCNYTNYTNCCNGCNGAY540    ATGGCNMGNGTNGTNYTNWSNGTNYTNTTYYTNGTNYTNYTNTTYTGGTGGYTNWSNAAR600    MGNMGNATHGAYYTNCCNATHACNGCNMGNYTNWSNMGNGAYWSNGAYYTNGCNACNYTN660    WSNACNYTNGCNTGGTGYTTYGGNACNGCNGCNATHWSNGGNGTNYTNGAYYTNWSNCCN720    GCNTAYGGNGCNTTYYTNGGNGGNGTNGTNYTNGGNAAYWSNGCNCARMGNGAYATGYTN780    YTNAARMGNGCNCARCCNATHGGNWSNGTNYTNYTNATGGTNTTYTTYYTNWSNATHGGN840    YTNYTNYTNGAYTTYAARTTYATHTGGAARAAYYTNGGNACNGTNYTNACNYTNYTNGCN900    ATGGTNACNYTNTTYAARACNGCNYTNAAYGTNACNGCNYTNMGNYTNGCNMGNCARGAY960    TGGCCNWSNGCNTTYYTNGCNGGNGTNGCNYTNGCNCARATHGGNGARTTYWSNTTYYTN1020    YTNGCNGARACNGGNAARGCNGTNAARYTNATHWSNGCNCARGARACNAARYTNGTNGTN1080    GCNGTNACNGTNYTNWSNYTNGTNYTNWSNCCNTTYTGGYTNTTYACNATGMGNMGNATG1140    CAYMGNGTNGCNGCNGTNCAYGTNCAYWSNTTYMGNGAYYTNGTNACNMGNYTNTAYGGN1200    GAYGARGCNMGNGCNTTYGCNMGNACNGCNMGNMGNGCNMGNGTNYTNGTNMGNMGNGGN1260    WSNTGGMGNGAYGAYCCNAAYGCNGGNCCNGGNWSNGGNATH1302    __________________________________________________________________________

What is claimed is:
 1. A fusion protein comprising:(a) a fragment of themagA gene product, the fragment comprising the amino acid sequence shownas residues 7-380 of SEQ ID NO: 2, (b) the amino acid sequence of afirst protein of interest, and (c) optionally, the amino acid sequenceof a second protein of interest; wherein the first and optional secondproteins of interest are the same or different, and wherein the firstand optional second proteins of interest are joined to the N- orC-terminal of the fragment of the magA gene product.
 2. A protein-boundmagnetic particle comprising a magnetic particle, an organic membranecovering the surfgace of the magnetic particle, and a fusion proteinaccording to claim 1 bound to the organic membrane.
 3. A protein-boundmagnetic particle according to claim 2, wherein either or both of thefirst and second proteins of interest is independently selected from thegroup consisting of an immunoglobulin, a binding protein, and an enzyme.4. A DNA molecule encoding a fusion protein according to claim
 1. 5. Aplasmid comprising the nucleotide sequence of a DNA molecule accordingto claim
 4. 6. A transformed host magnetic bacterium comprising aplasmid according to claim
 5. 7. A transformed host magnetic bacteriumaccording to claim 6, wherein the bacterium belongs to the speciesMagnetospirillum or Desulfovibrio.
 8. A process for producing aprotein-bound magnetic particle comprising a magnetic particle, anorganic membrane covering the surface of the magnetic particle, and afusion protein,the process comprising cultivating a transformed hostmagnetic bacterium according to claim 6 under conditions suitable toeffect expression of the DNA molecule contained in the plasmid and toeffect synthesis of the protein-bound magnetic particle by thetransformed host magnetic bacterium.
 9. A process according to claim 8,further comprising recovering the protein-bound magnetic particle bymagnetic separation.
 10. A fusion protein comprising:(a) a fragment ofthe magA gene product, the fragment comprising the amino acid sequenceshown as residues 7-380 of SEQ ID NO: 2, and (b) the amino acid sequenceof a protein of interest selected from the group consisting of animmunoglobulin, a binding protein, and an enzyme; wherein the protein ofinterest is joined to the N- or C-terminal of the fragment of the magAgene product.
 11. A protein-bound magnetic particle comprising amagnetic particle, an organic membrane covering the surface of themagnetic particle, and a fusion protein according to claim 10 bound tothe organic membrane.
 12. A DNA molecule encoding a fusion proteinaccording to claim
 10. 13. A plasmid comprising the nucleotide sequenceof a DNA molecule according to claim
 12. 14. A transformed host magneticbacterium comprising a plasmid according to claim
 13. 15. A transformedhost magnetic bacterium according to claim 14, wherein the bacteriumbelongs to the species Magnetospirillum or Desulfovibrio.
 16. A processfor producing a protein-bound magnetic particle comprising a magneticparticle, an organic membrane covering the surface of the magneticparticle, and a fusion protein,the process comprising cultivating atransformed host magnetic bacterium according to claim 14 underconditions suitable to effect expression of the DNA molecule containedin the plasmid and to effect synthesis of the protein-bound magneticparticle by the transformed host magnetic bacterium.
 17. A processaccording to claim 16, further comprising recovering the protein-boundmagnetic particle by magnetic separation.